The invention relates to protective coatings for electronic assemblies on a circuit board, and to methods for their manufacture.
Protective coatings for electronic assemblies are described, e.g., by James J. Licari, "Plastic Coatings for Electronics", McGraw-Hill Book Company, New York. Such coatings serve to protect electronic modules on circuit boards against ambient factors such as humidity and contamination which may lead to corrosion of circuit elements, to attendant changes in circuit characteristics, and to diminished isolation resistance between conductor paths of a printed circuit or between the contacts of circuit elements.
Known protective coatings are typically applied by dipping, casting, varnishing, spraying or painting with a suitable plastic material or an artificial resin. Disadvantageously, such applications are slow to dry and harden. Also, they are difficult to apply uniformly especially over sharp edges and points, and may require repeated application and inversion of the circuit board during drying. Complicated dosage control devices are required whose maintenance is costly and which require frequent cleaning, localized application leaving certain components uncovered is difficult, and resulting coatings typically have pores and are not sufficiently and uniformly dense.
According to Japanese Patent Document JP-A-60,209,884 in the name of Toshiba KK (see Patent Abstracts of Japan, Vol. 10, No. 72), the reliability of electrical connections in printed circuits on circuit boards is improved upon application of thermoplastic layers on front- and back-sides of a circuit board, e.g., layers of polyvinylchloride or the like, followed by melting to adhere the layers in forming integrated-circuit cards. Disadvantageously, circuits covered by this method are no longer accessible, i.e., a resulting integrated-circuit card does not permit access to circuit components for carrying out measurements, and circuit modules can no longer be replaced.